The present invention relates to polynucleotides and polypeptides of acetyl-CoA carboxylase in cyanobacteria and plants. Polynucleotides encoding acetyl-CoA carboxylase have use in conferring herbicide resistance and in determining the herbicide resistance of plants in a breeding program.
Acetyl-CoA carboxylase (ACC) is the first enzyme of the biosynthetic pathway to fatty acids. It belongs to a group of carboxylases that use biotin as cofactor and bicarbonate as a source of the carboxyl group. ACC catalyzes the addition of CO2 to acetyl-CoA to yield malonyl-CoA in two steps as shown below.
BCCP+ATP+HCOxe2x88x923xe2x86x92BCCP-CO2+ADP+Pixe2x80x83xe2x80x83(1)
BCCP-CO2+Acetyl-CoAxe2x86x92BCCP+malonyl-CoAxe2x80x83xe2x80x83(2)
First, biotin becomes carboxylated at the expense of ATP. The carboxyl group is then transferred to Ac-CoA [Knowles, 1989]. This irreversible reaction is the committed step in fatty acid synthesis and is a target for multiple regulatory mechanisms. Reaction (1) is catalyzed by biotin carboxylase (BC); reaction (2) by transcarboxylase (TC); BCCP=biotin carboxyl carrier protein.
ACC purified from E.coli contains three distinct, separable components.: biotin carboxylase (BC), a dimer of 49-kD monomers, biotin carboxyl carrier protein (BCCP) a dimer of 17-kD monomers and transcarboxylase (TC), a tetramer containing two each of 33-kD and 35-kD subunits. The biotin prosthetic group is covalently attached to the xcex3-amino group of a lysine residue of BCCP. The primary structure of E.coli BCCP and BC is known (fabE and fabG genes, respectively, have been cloned and sequenced) [Alix, 1989; Maramatsu, et al., 1989; Li, et al., 1992]. In bacteria, fatty acids are primarily precursors of phospholipids rather than storage fuels, and so ACC activity is coordinated with cell growth and division.
Rat and chicken ACC consist of a dimer of about 265 kD (rat has also a 280 kD isoform) subunits that contains all of the bacterial enzyme activities. Both mammalian and avian ACC are cytoplasmic enzymes and their substrate is transported out of mitochondria via citrate. ACC content and/or activity varies with the rate of fatty acid synthesis or energy requirements in different nutritional, hormonal and developmental states. ACC mRNA is transcribed using different promoters and can be regulated by alternative splicing. ACC catalytic activity is regulated allosterically by a number of metabolites and by reversible phosphorylation of the enzyme. The primary structure of rat and chicken enzymes, and the primary structure of the 5xe2x80x2-untranslated region of mRNA have been deduced from cDNA sequences [Lopez-Casillas, et al., 1988; Takai, et al., 1988]. The primary structure of yeast ACC has also been determined [Feel, et al., 1992].
Studies on plant ACC are far less advanced [Harwood, 1988]. It was originally thought that plant ACC consisted of low molecular weight dissociable subunits similar to those of bacteria. Those results appeared to be due to degradation of the enzyme during purification. More recent results indicate that the wheat enzyme, as well as those from parsley and rape, are composed of two about 220 kD monomers, similar to the enzyme from rat and chicken [Harwood, 1988; Egin-Buhler, et al., 1983; Wurtelle, et al., 1990; Slabas, et al., 1985]. The plant ACC is located entirely in the stroma of plastids, where all plant fatty acid synthesis occurs. No plant gene encoding ACC has been reported to date. The gene must be nuclear because no corresponding sequence is seen in the complete chloroplast DNA sequences of tobacco, liverwort or rice. ACC, like the vast majority of chloroplast proteins which are encoded in nuclear DNA, must be synthesized in the cytoplasm and then transported into the chloroplast, probably requiring a chloroplast transport sequence. Although the basic features of plant ACC must be the same as those of prokaryotic and other eucaryotic ACCs, significant differences can be also expected due, for example, to differences in plant cell metabolism and ACC cellular localization.
Structural similarities deduced from the available amino acid sequences suggest strong evolutionary conservation among biotin carboxylases and biotin carboxylase domains of all biotin-dependent carboxylases. On the contrary, the BCCP domains show very little conservation outside the sequence E(A/V)MKM (lysine residue is biotinylated) which is found in all biotinylated proteins including pyruvate carboxylase and propionyl-CoA carboxylase [Knowles, 1989; Samols, et al., 1988]. It is likely that the three functional domains of ACC located in E.coli on separate polypeptides are present in carboxylases containing two (human propionyl-CoA carboxylase) or only one (yeast pyruvate carboxylase, mammalian, avian and probably also plant ACC) polypeptide as a result of gene fusion during evolution.
Several years ago it was shown that aryloxyphenoxypropionates and cyclohexanediones, powerful herbicides effective against monocot weeds, inhibit fatty acid biosynthesis in sensitive plants. Recently it has been determined that ACC is the target enzyme for both of these classes of herbicide. Dicotyledonous plants are resistant to these compounds, as are other eukaryotes and prokaryotes. The mechanisms of inhibition and resistance of the enzyme are not known [Lichtenthaler, 1990].
It has occurred to others that the evolutionary relatedness of cyanobacteria and plants make the former useful sources of cloned genes for the isolation of plant cDNAs. For example, Pecker et al used the cloned gene for the enzyme phytoene desaturase, which functions in the synthesis of carotenoids, from cyanobacteria as a probe to isolate the cDNA for that gene from tomato [Pecker, et al., 1992].
In one aspect the present invention provides an isolated and purified polynucleotide of from about 1350 to about 40,000 base pairs that encodes a polypeptide having the ability to catalyze the carboxylation of a biotin carboxyl carrier protein of a cyanobacterium. Preferably, that polypeptide is a subunit of acetyl-CoA carboxylase and participates in the carboxylation of acetyl-CoA. In a preferred embodiment, a cyanobacterium is Anabaena or Synechococcus. The biotin carboxyl carrier protein preferably includes the amino acid residue sequence shown in SEQ ID NO:111 or a functional equivalent thereof.
In another preferred embodiment, the polypeptide has the amino acid residue sequence of FIG. 1 or FIG. 2. The polynucleotide preferably includes the DNA sequence of SEQ ID NO:1, the DNA sequence of SEQ ID NO:1 from about nucleotide position 1300 to about nucleotide position 2650 or the DNA sequence of SEQ ID NO:5.
In another aspect, the present invention provides an isolated and purified polynucleotide of from about 480 to about 40,000 base pairs that encodes a biotin carboxyl carrier protein of a cyanobacterium and, preferably Anabaena. The biotin carboxyl carrier protein preferably includes the amino acid residue sequence of SEQ ID NO:111 and the polynucleotide preferably includes the DNA sequence of SEQ ID NO:110.
Another polynucleotide provided by the present invention encodes a plant polypeptide having the ability to catalyze the carboxylation of acetyl-CoA. A plant polypeptide is preferably (1) a monocotyledonous plant polypeptide such as a wheat, rice, maize, barley, rye, oats or timothy grass polypeptide or (2) a dicotyledonous plant polypeptide such as a soybean, rape, sunflower, tobacco, Arabiodopsis, petunia, Canola, pea, bean, tomato, potato, lettuce, spinach, alfalfa, cotton or carrot polypeptide. Preferably, that polypeptide is a subunit of ACC and participates in the carboxylation of acetyl-CoA.
Such a polynucleotide preferably includes the nucleotide sequence of SEQ ID NO:108 and encodes the amino acid residue sequence of SEQ ID NO:109.
In yet another aspect, the present invention provides an isolated and purified DNA molecule comprising a promoter operatively linked to a coding region that encodes (1) a polypeptide having the ability to catalyze the carboxylation of a biotin carboxyl carrier protein of a cyanobacterium, (2) a biotin carboxyl carrier protein of a cyanobacterium or (3) a plant polypeptide having the ability to catalyze the carboxylation of acetyl-CoA, which coding region is operatively linked to a transcription-terminating region, whereby said promoter drives the transcription of said coding region.
In another aspect, the present invention provides an isolated polypeptide having the ability to catalyze the carboxylation of a biotin carboxyl carrier protein of a cyanobacterium such as Anabaena or Synechococcus. Preferably a biotin carboxyl carrier protein includes the amino acid sequence of SEQ ID NO:111 and the polypeptide has the amino acid residue .sequence of FIG. 1 or FIG. 2 (SEQ ID NO:5 and SEQ ID NO:6).
The present invention also provides (1) an isolated and purified biotin carboxyl carrier protein of a cyanobacterium such as Anabaena, which protein includes the amino acid residue sequence of SEQ ID NO:111 and (2) an isolated and purified plant polypeptide having a molecular weight of about 220 kD, dimers of which have the ability to catalyze the carboxylation of acetyl-CoA.
In yet another aspect, the present invention provides a process of increasing the herbicide resistance of a monocotyledonous plant comprising transforming the plant with a DNA molecule comprising a promoter operatively linked to a coding region that encodes a herbicide resistant polypeptide having the ability to catalyze the carboxylation of acetyl-CoA, which coding region is operatively linked to a transcription-terminating region, whereby the promoter is capable of driving the transcription of the coding region in a monocotyledonous plant.
Preferably, a polypeptide is an acetyl-CoA carboxylase enzyme and, more preferably, a dicotyledonous plant acetyl-CoA carboxylase. In a preferred embodiment, a coding region includes the DNA sequence of SEQ ID NO:108 and a promoter is CaMV35.
The present invention also provides a transformed plant produced in accordance with the above process as well as a transgenic plant and a transgenic plant seed having incorporated into its genome a transgene that encodes a herbicide resistant polypeptide having the ability to catalyze the carboxylation of acetyl-CoA.
In yet another aspect, the present invention provides a process of altering the carboxylation of acetyl-CoA in a cell comprising transforming the cell with a DNA molecule comprising a promoter operatively linked to a coding region that encodes a plant polypeptide having the ability to catalyze the carboxylation of acetyl-CoA, which coding region is operatively linked to a transcription-terminating region, whereby the promoter is capable of driving the transcription of the coding region in the cell.
In a preferred embodiment, a cell is a cyanobacterium or a plant cell and a plant polypeptide is a monocotyledonous plant acetyl-CoA carboxylase enzyme such as wheat acetyl-CoA carboxylase enzyme. The present invention also provides a transformed cyanobacterium produced in accordance with such a process.
The present invention still further provides a process for determining the inheritance of plant resistance to herbicides of the aryloxyphenocypropionate or cyclohexanedione class, which process comprises the steps of:
(a) measuring resistance to herbicides of the aryloxyphenocypropionate or cyclohexanedione class in a parental plant line and in progeny of the parental plant line;
(b) purifying DNA from said parental plant line and the progeny;
(c) digesting the DNA with restriction enzymes to form DNA fragments;
(d) fractionating the fragments on a gel;
(e) transferring the fragments to a filter support;
(f) annealing the fragments with a labelled RFLP probe consisting of a DNA molecule that encodes acetyl-CoA carboxylase or a portion thereof; and
(g) detecting the presence of complexes between the fragments and the RFLP probe; and
(h) correlating the herbicide resistance of step (a) with the complexes of step (g) and thereby the inheritance of herbicide resistance.
Preferably, the acetyl-CoA carboxylase is a dicotyledonous plant acetyl-CoA carboxylase enzyme or a mutated monocotyledonous plant acetyl-CoA carboxylase that confers herbicide resistance or a hybrid acetyl-CoA carboxylase comprising a portion of a dicotyledonous plant acetyl-CoA carboxylase, a portion of a dicotyledonous plant acetyl-CoA carboxylase or one or more domains of a cyanobacterial acetyl-CoA carboxylase.
In still yet another aspect, the present invention provides a process for identifying herbicide resistant variants of a plant acetyl-CoA carboxylase comprising the steps of:
(a) transforming cyanobacteria with a DNA molecule that encodes a monocotyledonous plant acetyl-CoA carboxylase enzyme to form transformed cyanobacteria;
(b) inactivating cyanobacterial acetyl-CoA carboxylase;
(c) exposing the transformed cyanobacteria to a herbicide that inhibits acetyl-CoA carboxylase activity;
(d) identifying transformed cyanobacteria that are resistant to the herbicide; and
(e) characterizing DNA that encodes acetyl-CoA carboxylase from the cyanobacteria of step (d).